Issues with group calls on iphone 7

Over the last 3 months I have found that both I and others I know using iPhones are not able to make conference (group) calls reliably. For example, last Tuesday I tested the facility with making 5 calls (4 iphone, one Android, mix of Optus and Telstra) and it worked fine. On Thursday I wasn't able to connect more than 2 phones without the successful calls being dropped and/or the new calls giving "call failed". I have hung up and restarted calls. I have tried turning the phone off then on again. I am using iOS 11.3.1 on an iPhone 7, 128 Gb, 90 GB available. Using Telstra as service provider in a capital city. I have been having this problem repeatedly. I have spoken to Telstra and they assure me that there absolutely no Telstra related issue of limit that would cause this. Any suggestions much appreciated.